It is a well known fact to physicists and engineers alike that a body in motion comes to a halt in a shorter distance when decelerated to a gradual stop as opposed to skidding. When the moving object is a vehicle, factors such as directional control and braking distance is negatively affected when the vehicle enters a skidding condition during braking. That is to say, directional control of the vehicle, i.e., steerability, is significantly reduced while braking distance, i.e., the distance needed to bring the vehicle to a complete stop, is significantly increased. Therefore, modem vehicle manufactures have incorporated ABS into the vehicle's normal braking system to prevent wheels from locking, i.e., preventing the vehicle from entering into a skidding condition, when the vehicle brakes are activated by the driver during conditions when skidding is most likely to occur, e.g., slippery road conditions, to allow the driver to retain directional control of the vehicle.
FIG. 1 shows an example of a typical braking circuit used by ABS/TCS. In a normal braking mode, the braking process is actuated when a driver applies steady pressure on brake pedal 20 causing the braking fluid in master cylinder 22 via vacuum boost 24 to travel through the brake circuit to press against brake pad 26 installed on wheel 28 to initiate deceleration of vehicle. Under normal road conditions, the frictional force created by the decelerating wheels against the road surface, i.e., traction, is greater than the frictional forces of the brake pads against each wheel such that the vehicle decelerates gradually without skidding. However, under slippery road conditions or in sudden braking conditions, the frictional force of the wheel pads on the wheels may be greater than the traction of the wheels on the road causing the wheels to lock and skidding to occur.
A typical antilock braking system includes a controller(not shown) which monitors, among other parameters, vehicle speed (.upsilon..sub.vehicle) and wheel speed(.upsilon..sub.wheel) of each wheel. During operation of the vehicle, the ABS controller constantly compares .upsilon..sub.wheel with a reference speed (.upsilon..sub.reference) .upsilon..sub.reference is a predetermined threshold of .upsilon..sub.wheel, e.g., a percentage of .upsilon..sub.vehicle, at which a wheel begins to approach the unstable area of a .mu.-adhesion/slip curve as indicated by line .lambda. in FIG. 2 or as shown in FIG. 3. Under normal braking conditions, .upsilon..sub.vehicle and .upsilon..sub.wheel decelerate in substantially a linear manner such that .upsilon..sub.wheel of each wheel does not drop below .upsilon..sub.reference during the duration of the braking process, i.e., wheels do not lock. Therefore ABS is not activated during such a condition.
However, as shown in FIG. 2, if .upsilon..sub.wheel of a wheel drops below .upsilon..sub.reference during a braking process, it is likely that the monitored wheel is about to fall below the slip switching threshold .lambda., i.e., wheel lock. When such a condition is detected, the ABS controller takes control of the brake system in order to maintain the unstable wheel in the stable region of operation, i.e., prevent from lock up. As shown in FIG. 1, a typical antilock braking system includes pump assembly 30. When a tendency for wheel lock is detected on wheel 28, the ABS controller (not shown) actuates normally open ("NO") valve 32 to close (pressure hold) and normally closed ("NC") 34 valve to open (pressure dump). Depending on the pressure existing in the now isolated wheel brake circuit, ABS controller activates pump 30 to remove a certain amount of brake fluid from low pressure accumulator 36 thereby relieving brake pressure off of wheel 28 such that the wheel starts to re-accelerate again. By constantly comparing .upsilon..sub.wheel against .upsilon..sub.reference, as well as other parameters, such as peripheral wheel acceleration or speed, the ABS controller activates and deactivates return pump 30 to control the amount of brake pressure on wheel 28 to prevent wheel lock during braking.
In a typical traction control system, the controller is constantly monitoring .upsilon..sub.wheel to .upsilon..sub.reference to control wheel slip similar to ABS. However, unlike ABS which detects wheel slip during deceleration of a vehicle, TCS detects wheel slip during acceleration of the vehicle. That is to say, the controller in TCS applies braking pressure to the slipping wheels as opposed to the controller in ABS which relieves braking pressure to the slipping wheels.
Under normal road conditions, the accelerating wheel transfers its force to accelerate the vehicle by virtue of traction of the wheel to the surface of the road. However, under wheel slip conditions, the acceleration force of the wheel is greater than the frictional force of the wheel on the surface of the road, i.e., low traction, resulting in a spinning wheel. In order to bring the slipping wheel into a stable operating condition, the TCS controller applies brake pressure to the slipping wheel to slow it down such that the frictional force of the wheel to the surface of the road, i.e., traction, is greater than the force of the spinning wheel allowing the wheel to transfer the acceleration force to the vehicle.
Typically, the controller of an ABS and the controller of a TCS are one in the same although dedicated controller for ABS and TCS may be used. Similarly, the pump that relives braking pressure in ABS mode may be used to provide braking pressure in TCS mode, although dedicated pumps may be used for ABS and TCS respectively. In either case, the activation of the pump is a major source of driver discomfort in the prior art systems.
The noise produced by the pump is so loud that many drivers are startled and disconcerted when the pumps are activated unless the driver is already familiar with ABS/TCS operations. Furthermore, when the motor activates and pumps the brake fluid during ABS/TCS control mode, the driver usually feels the brake pedal pulsing under his or her feet due to the brake fluid slamming against actuating valves in synchronization with the motor noise. Such an occurrence may distract the driver as well as cause psychological distress for fear of possible brake failure or some type of vehicle break down.
In addition, some prior art systems utilize a pressure sensor associated with the brake system to monitor the pressure in the brake circuit. However, some of these pressure sensors are sensitive to vibrations needing costly repairs to recalibrate or replace the pressure sensors. Furthermore, the cost of these pressure sensors themselves are expensive, driving up the cost of manufacturing of vehicles with this type of system.